Functional model for catecholase-like activity: A mechanistic approach with manganese(III) complexes of salen type Schiff base ligands

Article abstract of DOI:10.1016/j.molcata.2012.08.024

Three new Mn(III) complexes [MnL¹(OOCH)(OH₂)] (1), [MnL²(OH₂)₂][Mn₂L₂ ²(NO₂)₃] (2) and [Mn₂L ₂¹(NO₂)₂] (3) (where H ₂L¹ = H₂Me₂Salen = 2,7-bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene and H₂L² = H₂Salpn = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene) have been synthesized. X-ray crystal structure analysis reveals that 1 is a mononuclear species whereas 2 contains a mononuclear cationic and a dinuclear nitrite bridged (μ-1κO:2κO′) anionic unit. Complex 3 is a phenoxido bridged dimer containing terminally coordinated nitrite. Complexes 1-3 show excellent catecholase-like activity with 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate. Kinetic measurements suggest that the rate of catechol oxidation follows saturation kinetics with respect to the substrate and first order kinetics with respect to the catalyst. Formation of bis(μ-oxo)dimanganese(III,III) as an intermediate during the course of reaction is identified from ESI-MS spectra. The characteristic six line EPR spectra of complex 2 in the presence of 3,5-DTBC supports the formation of manganese(II)-semiquinonate as an intermediate species during the catalytic oxidation of 3,5-DTBC.

Full text of DOI:10.1016/j.molcata.2012.08.024

Journal of Molecular Catalysis A: Chemical 365 (2012) 154–161
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Functional model for catecholase-like activity: A mechanistic approach with
manganese(III) complexes of salen type Schiff base ligands
Piya Setha,1, Michael G.B. Drewb, Ashutosh Ghosha,∗
a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
b School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK
a r t i c l e i n f o
a b s t r a c t
Three new Mn(III) complexes [MnL1(OOCH)(OH2)] (1), [MnL2(OH2)2][Mn2L22(NO2)3] (2) and
[Mn2L21(NO2)2] (3) (where H2L1 = H2Me2Salen = 2,7-bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene and
H2L2 = H2Salpn = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene) have been synthesized. X-ray
crystal structure analysis reveals that 1 is a mononuclear species whereas 2 contains a mononuclear
cationic and a dinuclear nitrite bridged (␮-1ꢀO:2ꢀOꢀ) anionic unit. Complex 3 is a phenoxido bridged
dimer containing terminally coordinated nitrite. Complexes 1–3 show excellent catecholase-like activity
with 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate. Kinetic measurements suggest that the rate
of catechol oxidation follows saturation kinetics with respect to the substrate and first order kinetics
with respect to the catalyst. Formation of bis(␮-oxo)dimanganese(III,III) as an intermediate during the
course of reaction is identified from ESI-MS spectra. The characteristic six line EPR spectra of complex 2
in the presence of 3,5-DTBC supports the formation of manganese(II)-semiquinonate as an intermediate
species during the catalytic oxidation of 3,5-DTBC.
Article history:
Received 3 April 2012
Received in revised form 23 August 2012
Accepted 25 August 2012
Available online 1 September 2012
Keywords:
Manganese(III)
Schiff base
Crystal structure
Catecholase activity
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
consists of a hydroxo-bridged dicopper(II) center. Therefore,
structure–activity relationships for the catechol oxidation reac-
tion are mainly investigated with Cu(II) complexes [11]. From this
survey, it can be said that the rate of catechol oxidation reac-
tions catalyzed by Cu(II) complexes is a composite effect of several
parameters such as: M–M distance, flexibility of the ligand, type
of exogenous ligand and coordination geometry around the metal
ion. Besides the Cu(II) containing species, some complexes of Mn(II)
[12–14], Mn(III) [15,16], Mn(IV) [17,18] have also being reported
to perform catecholase mimicking activity. In these complexes dif-
ferent types of ligands e.g. tetra-dentate tripodal ligands, several
pyridine derivatives, azametallacrown and compartmental ligands
have been utilized. However, the number of such studies is far less
in comparison to Cu(II) complexes, although the manganese(III)
complexes play very important roles in biological systems e.g. in
many metalloenzymes, redox and non-redox proteins [19,20], and
are used as catalysts in olefin epoxidation reactions [21].
In order to have clearer insight into the catecholase activ-
ity of manganese complexes we have chosen complexes
of salen-type Schiff base ligands (Scheme 1), and synthe-
sized three new Mn(III) complexes, [MnL1(OOCH)(OH2)] (1),
[MnL2(OH2)2][Mn2L22(NO2)3] (2) and [Mn2L21(NO2)2] (3). Their
characterization, crystal structure and catecholase activity are
reported in this paper. All three complexes exhibit substantial cat-
echol oxidation with 3,5-di-tert-butylcatechol (3,5-DTBC) as the
substrate in acetonitrile solvent. The phenoxido bridged dinuclear
Metalloenzymes that activate molecular oxygen have received
great attention as potential catalysts for specific oxidation reactions
and also for the development of efficient small molecule catalysts
[1–6]. The oxidation of organic substrates with molecular oxygen
under mild conditions is of growing interest for industrial and syn-
thetic processes [7]. As a result synthesis and reactivity studies
of transition metal complexes, as model compounds for metal-
loenzymes with oxidase activity have been undertaken by various
groups in order to develop bio-inspired catalysts for oxidation reac-
tions [8–10]. One of the major enzymes that play a key role in these
reactions is catechol oxidase (CO), a lesser known member of the
type-III copper proteins. This enzyme belongs to the polyphenol
oxidases which oxidize phenolic compounds to the corresponding
quinones in the presence of oxygen. Quinones are highly reactive
compounds that autopolymerize to brown polyphenolic catechol
melanins. Some higher plants utilize this process to resist diseases
and to protect themselves from pathogens or insects.
Most abundant functional mimics of CO are mono- or dinu-
clear Cu(II) complexes [9,10] as the native form of this enzyme
∗
Corresponding author. Tel.: +91 9433344484; fax: +91 3323519755.
E-mail address: ghosh 59@yahoo.com (A. Ghosh).
Tel.: +91 9433344484; fax: +91 33 2351 9755.
1
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.024

P. Seth et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 154–161
155
(CH2)n
started to separate on stirring; the solids were filtered after about
2 h and washed with diethyl ether, and then redissolved in CH3CN.
Layering of these brown solutions with Et2O gave well formed X-ray
quality single crystals.
R
R
N
N
Complex 2: Yield: 0.846 g (72%) Anal Calc. for
C
51H52Cl0.38Mn3N8.62O13.23: C, 51.92; H, 4.44; N, 10.68; Found:
OH
HO
C, 51.76; H, 4.31; N, 10.51. IR (KBr pellet): ꢁ(C N), 1612 cm−1
ꢁs(NO2), 1295 cm−1, ꢁas(NO2), 1213 cm−1, ꢁ(H2O), 3395 cm−1
UV–vis (CH3CN) ꢂmax (nm), ε(M−1 cm−1): 388 (17,549).
,
.
H2L1 (n=0, R=CH3)
H2L2 (n=1, R=H)
Complex 3: Yield: 0.584 g (75%) Anal Calc. for
C36H36Cl1.02Mn2N4.98O5.96: C, 54.69; H, 4.59; N, 10.63; Found:
Scheme 1. Schiff base ligand H2L.
C, 54.52; H, 4.69; N, 10.47. IR (KBr pellet): ꢁ(C N), 1589 cm−1
,
ꢁs(NO2), 1313 cm−1, ꢁas(NO2), 1234 cm−1. UV–vis (CH3CN) ꢂmax
(nm), ε(M−1 cm−1): 397 (9013).
Mn(III) compounds with this kind of N2O2 donor Schiff base lig-
ands are well known for their interesting magnetic properties [22]
specially for their potential use as single molecule magnets [23].
A nitrito bridged complex of such ligand also exhibited the sign of
SMM behavior [24]. However, the catecholase activity of Mn(III)
complexes with salen type di-Schiff bases have not been explored
till date. We show here that these complexes are very good cata-
lysts for catechol oxidation reaction and try to explain the plausible
mechanisms for the same. A manganese(II)-semiquinonate inter-
mediate which is identified through EPR measurements is thought
to be responsible for this oxygenation reaction. ESI-MS positive
spectra also signify the presence of a bis(␮-oxo)dimanganese(III,III)
intermediate during the aerobic oxidation of catechol.
2.2. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed using a Perkin-Elmer 240C elemental analyzer. IR spectra
in KBr (4000–500 cm−1) were recorded using a Perkin-Elmer
RXI FT-IR spectrophotometer. Electronic spectra in acetonitrile
(600–300 nm) were recorded in a Hitachi U-3501 spectrophotome-
ter. The electro spray ionization mass (ESI-MS positive) spectra
were recorded on a MICROMASS Q-TOF mass spectrometer. The
X-band (9.1 GHz) EPR spectra were recorded using a JEOL JES-FA
200 instrument at liquid nitrogen temperature (77 K) in acetonitrile
solvent.
2. Experimental
2.2.1. Catalytic oxidation of 3,5-DTBC
2.1. Synthesis of the complexes
In order to examine the catecholase activity of the complexes,
a 10−5 M solution of 1–3 in acetonitrile solvent was treated
with 100 equiv. of 3,5-di-tert-butylcatechol (3,5-DTBC) under aer-
obic conditions at room temperature. Absorbance vs. wavelength
(wavelength scans) of these solutions were recorded at a regular
time interval of 5 min in the wavelength range 300–500 nm. To
determine the dependence of rate on substrate concentration and
various kinetic parameters, a 10−5 M solution of these complexes
was treated with at least 10 equiv. of substrate so as to maintain
the pseudo first order condition. The reaction was followed spec-
trophotometrically by monitoring the increase in the absorbance at
400 nm (Quinone band maxima) as a function of time (time scan).
o-Hydroxy acetophenone, 1,2-ethanediamine, salicylaldehyde
and 1,3-diaminopropane were purchased from Lancaster and were
of reagent grade. They were used without further purification.
2.1.1. Synthesis of the Schiff base ligands, H2L1 and H2L2
The two di-Schiff-base ligands, H2L1 and H2L2 (Scheme 1), were
prepared by standard methods [25]. Briefly, 2 mM of diamine {1,2-
ethanediamine (0.12 mL) or 1,3-diaminopropane (0.16 mL)} was
mixed with 4 mM of the required aldehyde {o-hydroxy acetophe-
none (0.48 mL) or salicylaldehyde (0.41 mL)} in methanol solvent.
The resulting mixture was refluxed for ∼1.5 h, and allowed to cool.
The yellow solutions were used directly for complex formation.
2.3. Crystallographic data collection and refinement
2.1.2. Synthesis of complex [MnL1(OOCH)(OH2)] (1)
Crystal data for the three complexes 1–3 are given in the
supporting information, Table ST1. 4021, 14044 independent
reflections data were collected with MoK␣ radiation for 1 and 2
at 150 K using the Oxford Diffraction X-Calibur CCD System. The
crystals were positioned at 50 mm from the CCD. 321 frames were
measured with a counting time of 10 s. Data analyses were carried
out with the CrysAlis program [26]. 2556 independent reflections
data for 3 were collected with MoK␣ radiation at room temperature
using the Bruker-AXS SMART APEX II diffractometer equipped with
A methanolic solution (10 mL) of H2L1 (2 mM) was added to
a methanolic solution (5 mL) of Mn(ClO4)2·6H2O (0.722 g, 2 mM)
with constant stirring. After ∼10 min an aqueous solution of
HCOONa (0.136 g, 2 mM) was added to the solution, followed by
triethyl amine (0.56 mL, 4 mM). The solution was further stirred
for half an hour. A dark brown crystalline compound separated on
stirring. It was filtered. On slow evaporation of the solvent from the
filtrate dark brown, X-ray quality single crystals were obtained.
Complex 1: Yield: 0.313 g (78%) Anal Calc. for C19H21Mn1N2O5:
C, 55.35; H, 5.13; N, 6.79; Found: C, 55.47; H, 5.07; N, 6.62. IR
(KBr pellet): ꢁ(C N), 1590 cm−1, ꢁas(C O), 1537 cm−1, ␯s(C O),
1328 cm−1. UV–vis (CH3CN) ꢂmax (nm), ε(M−1 cm−1): 396 (9491).
˚
a graphite monochromator and MoK␣ (ꢂ = 0.71073 A) radiation.
The crystal was positioned at 60 mm from the CCD. 360 frames were
measured with a counting time of 10 s. All three structures were
solved using direct methods with the Shelxs97 program [27]. Both
2 and 3 showed some disorder between one terminal nitrite and
chloride ions. In 2 this was treated via population parameters x and
1 − x, respectively with x refined to 0.62(1). In 3 similar refinement
converged to 0.49(3) and was subsequently fixed at 0.5. The non-
hydrogen atoms were refined with anisotropic thermal parameters.
The hydrogen atoms bonded to carbon were included in geomet-
ric positions and given thermal parameters equivalent to 1.2 times
(or 1.5 times for methyl groups) those of the atom to which they
were attached. The hydrogen atoms of the water molecules in 1 and
2.1.3. Synthesis of the complexes [MnL2(OH2)2][Mn2L22(NO2)3]
(2) and [Mn2L21(NO2)2] (3)
A methanolic solution (10 mL) of the required ligand H2L1
or H2L2 (2 mM) was added to a methanolic solution 5 mL of
MnCl2·4H2O (0.394 g, 2 mM) with constant stirring. To this solu-
tion triethylamine (0.56 mL, 4 mM) was added. After ca. 15 min a
H2O–MeOH mixture of NaNO2 (0.169 g, 2 mM) was added to the
mixture and stirred. The dark brown compound of both 2 and 3

156
P. Seth et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 154–161
2 were located in difference Fourier maps and refined with dis-
tance constraints. Absorption corrections were carried out using
the ABSPACK program [28] for 1 and 2 and the SADABS program
for 3 [29]. The structures were refined on F2 to R1 0.0756, 0.0909,
0.1061; wR2 0.1968, 0.1850, 0.3331 for 2903, 6142, 2126 data with
I > 2ꢃ(I).
OOCH
X
H3C
O
CH3
N
N
Mn
H3C
CH3
N
N
O
O
Mn
O
O
O
Mn
OH2
N
N
3. Results and discussion
(1)
CH3
H3C
X
3.1. Syntheses, IR and UV–vis spectra of the complexes
(3)
X= Cl (50%)
NO2 (50%)
1
was synthesized by the reaction of H2L1,
H
H
X
OH2
Compound
N
N
Mn(ClO4)2·6H2O, HCOONa, Et3N in 1:1:1:2 molar ratio in methanol
solvent. Compounds 2 and 3 were synthesized by the similar reac-
tion of H2L1 or H2L2, MnCl2·4H2O, NaNO2 and triethylamine in
1:1:1:2 molar ratios using H2O–MeOH (1:10) as solvent. The Mn(II)
ion was oxidized by the aerial oxygen to Mn(III) under the reac-
tion conditions and the donor atoms of the deprotonated ligand
(H2L) coordinate to form the equatorial plane of the Mn(III) site,
as is usual for the salen-type Schiff base complexes. In complex
1, the formate ion axially coordinates to the metal centre. On the
other hand, the nitrite ion present in the reaction mixture bridges
through ␮-1ꢀO:2ꢀOꢀ mode in 2 and terminally coordinates through
an oxygen atom in 3 to the Mn(III) centre (Scheme 2).
Mn
H
O
H
N
N
O
O
O
Mn
N
O
O
O
O
Mn
OH2
N
N
O
X= Cl (38%)
NO2 (62%)
N
H
H
(2)
O
Scheme 2. Complexes 1–3.
Besides elemental analysis, all the complexes were initially
characterized by IR spectra. All three complexes exhibit a sharp
mononuclear species [MnL1]+. Another peak at m/z = 436 indicates
the presence of mononuclear species [MnL1(OOCH)(CH3CN)H]+ (SI
Fig. S5). The spectrum of 2 shows base peak at m/z = 279 (100%)
which is well assignable to the ligand species [H3L2]+. Two peaks
at m/z = 335, 413 are due to the mononuclear species [(MnL2)]+
and [(MnL2)(CH3CN)2]+, respectively. Two other peaks at m/z = 854,
885 can be assigned to the dinuclear species [Mn2(L2)2(NO2)3Na2]+
and [Mn2(L2)2(NO2)2(CH3CN)3H]+, respectively. (SI Fig. S6) The
spectrum of 3 shows a base peak at m/z = 349 (100%) which fits
well with the mononuclear species [(MnL1)]+. Four other peaks
at m/z = 733, 743, 813 and 802 are probably due to the pres-
ence of [(MnL1)2Cl]+, [(MnL1)2NO2]+, [(MnL1)2(NO2)Cl]Na+ and
[(MnL1)2(NO2)2]Na+ species, respectively in the solution state (SI
Fig. S7).
band due to azomethine ꢁ(C N) at 1590, 1612 and 1589 cm−1
,
respectively. For 1, the strong band at 1537 cm−1 is likely to be due
to the antisymmetric and the band at 1328 cm−1 to the symmet-
ric stretching modes for the formate [24]. Bands due to ꢁs(NO2)
and ꢁas(NO2) are observed at 1295, 1213 cm−1 (for 2) and 1313,
1234 cm−1 (for 3) as reported earlier for O-coordinated nitrite [30].
One additional band corresponding to ꢁ(H2O) is observed at 3487
and 3395 cm−1 for 1 and 2, respectively. UV–vis spectra of these
complexes in acetonitrile solvent show a broad band at 396 (for
1), 388 (for 2) and 397 nm (for 3) corresponding to ligand to metal
charge transfer [24,31].
3.2. Electrospray ionization mass spectral study
We have recorded ESI-MS spectra of a 1:100 mixture of complex
2 and 3,5-DTBC in acetonitrile solvent, to gain an insight of this oxi-
dation reaction in solution state. The spectrum (SI Fig. S8) exhibits
a base peak at m/z = 243 (100%) corresponding to the quinone
sodium aggregate [3,5-DTBQ-Na]+. The peak at m/z = 774 corrob-
orates the formation of [Mn2L22O2(CH3CN)2] via a Mn O2 species
The electrospray ionization mass (ESI-MS positive) spectrum of
1–3 was recorded in acetonitrile solvent to rationalize the compo-
sition of the complex in solution state. The spectrum of 1 shows
base peak at m/z = 349 (100%) which is well assignable to the
Fig. 1. The structure of 1 with ellipsoids at 30% probability.

P. Seth et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 154–161
157
Fig. 2. Hydrogen bonded polymeric structure of 1; hydrogen atoms except H1, H2, and H3 have been excluded for clarity.
as an intermediate [32]. Another peak at m/z = 631 can be assigned
of the ligand in an equatorial plane and two water molecules
in axial positions. The four donor atoms N(19), N(23), O(11),
to the mononuclear species [MnL2O2(3,5-DTBSQ)(CH3CN)].
˚
O(31) are planar with a r.m.s. deviation of 0.018 A with the
˚
metal 0.002(2) A from the plane. Bond lengths in the equa-
3.3. Description of structures
torial plane are Mn(1) O(11) 1.882(3), Mn(1) O(31) 1.857(4),
˚
Mn(1) N(19) 2.017(5), Mn(1) N(23) 2.040(4) A and in axial pos-
The monomeric structure of 1 is shown in Fig. 1 together with
the atomic numbering scheme in the coordination sphere. Here
the ligand contains an ethylene link between the two nitrogen
atoms N(19) and N(22). The metal has a six-co-ordinate octahe-
dral environment being bonded to the four donor atoms of ligand
L1 in the equatorial plane and one water molecule and a formate
anion in axial positions. The four donor atoms in the equatorial
˚
itions Mn(1) O(1) 2.218(4), Mn(1) O(2) 2.283(4) A (Table ST2).
The angle between the two phenyl rings is 4.6(3) with the two rings
intersecting the equatorial planes at angles of 13.0(2) and 13.2(2),
respectively. Thus the conformation of the cation is similar to that
found in 1. The structure of the anion [Mn2L22(NO2)3]− shows two
metal atoms Mn(2) and Mn(3) in similar octahedral environments
bonding to the ligand L2 with four donor atoms N(49), N(53), O(41),
O(61) for Mn(2) and N(79), N(83), O(71), O(91) for Mn(3) in the
equatorial plane. In the axial positions of each metal atom there are
two nitrite anions, one terminal and one bridging. The two equato-
˚
rial planes show r.m.s. deviations of 0.079, 0.035 A with the metal
˚
atoms 0.043(2), 0.042(2) A from the plane. In the equatorial plane
Mn N distances are in the range 2.006(4) to 2.029(4) A and Mn O
distances are in the range 1.879(3) to 1.891(3) A. Distances in axial
˚
plane show a r.m.s. deviations of 0.045 A with the metal atom
˚
0.021(2) A from the plane. The two phenyl rings in the ligand inter-
sect the equatorial plane by angles of 17.8(2) and 16.0(2) with
an angle of 4.2(2) between them. Bond lengths in the equato-
˚
˚
rial plane are Mn(1) O(11) 1.847(3) A, Mn(1) O(31) 1.890(3) A,
˚
˚
Mn(1) N(19) 1.987(4) A, Mn(1) N(23) 1.987(4) A and in axial pos-
˚
˚
˚
itions Mn(1) O(1) 2.265(3) A, Mn(1) O(2) 2.195(3) A (Table ST2).
The two hydrogen atoms H(1) and H(2) on the water molecule form
donor intermolecular hydrogen bonds to the phenoxido oxygen
O(30) (3/2 − x, y + 1/2, z) and the non-coordinated oxygen atom
of formate, O(4) (x, y + 1, z) with dimensions O(1)· · ·O 2.944(5),
˚
2.684(5) A; O(1) H· · ·O 148, 132◦ and H· · ·O 2.17, 2.01 A, respec-
tively to result in a 1D chain along the b-axis (Fig. 2).
˚
˚
The structure of 2 contains discrete [MnL2(OH2)2]+ cations
and [Mn2L22(NO2)3]− anions as shown in Figs. 3 and 4, respec-
tively. Here the ligand contains three methylene groups between
the two nitrogen atoms N(19) and N(23) rather than the two
methylene groups that are found in 1. The structure of the
cation [MnL2(OH2)2]+ the metal in a six-coordinate octahedral
environment with the metal bonded to the four donor atoms
2
−
anion in 2 with ellipsoids at 30% prob-
Fig. 4. The structure of the [Mn2(NO2)3L2
]
Fig. 3. The structure of the [Mn(L2)(OH2)2]+ cation in 2 with ellipsoids at 30% prob-
ability.
ability. The structure is disordered with terminal nitrite O9, N11, O10 and chloride
(not shown) having refined occupation factors of 0.62(1), and 0.38(1), respectively.

158
P. Seth et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 154–161
Fig. 5. Hydrogen bonded polymeric structure of 2; hydrogen atoms, except those of the water molecules, namely H11, H12, H21, and H22, have been excluded for clarity.
˚
the axial sites consisting of an anion, disordered between nitrite
and chloride, and an oxygen atom of a second ligand. Bond lengths
in the equatorial plane are Mn(1) O(11) 1.854(9) A, Mn(1) O(30)
positions are Mn(2) O(6) 2.252(5), Mn(3) O(9) 2.121(7) A to ter-
minal nitrites and Mn(2) O(5) 2.236(4), Mn(3) O(3) 2.308(4) Å
to bridging nitrites. In the anion, the two ML2 moieties show dif-
ferent conformations to those found in the cation and in 1. Thus
around Mn(2) the two phenyl rings intersect at 44.1(2)◦ with the
rings intersecting the equatorial plane at 12.4(3) and 32.5(2). Com-
parable angles for the ligand around Mn(3) are 2.7(3), 30.9(2), and
30.9(2). The two equatorial planes intersect at 62.3(1).
The two water molecules in the cation form four donor inter-
molecular hydrogen bonds (Fig. 5). From O(1) the hydrogen atom
H(11) forms a hydrogen bond to an oxygen atom O(8) of nitrite
˚
˚
˚
˚
1.885(8) A, Mn(1) N(19) 2.014(10) A, Mn(1) N(22) 2.001(11) A
ꢀ
with axial bonds Mn(1) O(30)ꢀ ( = −x, 1 − y, −z) 2.520(9) A and
˚
˚
˚
Mn(1) Cl(1) 2.503(12) A or Mn(1) O(3) 2.09(4) A (Table ST2). The
donor atoms in the equatorial plane show an r.m.s. deviation of
0.060 A with the metal 0.184(6) A from the plane in the direction
of the disordered anion. The two phenyl rings intersect at 20.9(8)
and intersect the equatorial plane at angles at 8.2(8) and 28.0(6).
˚
˚
˚
3.4. Kinetics of the oxidation of 3,5-di-tert-butylcatechol
(x − 1, −y + 1/2, z + 1/2) with dimensions O· · ·O 2.869(8) A, O H· · ·O
169◦ and H· · ·O 2.03 A. The other hydrogen H(12) forms bifurcated
˚
The catecholase activity of complexes 1–3 was studied using
3,5-DTBC as a convenient model substrate [33], in air saturated
acetonitrile solvent at room temperature (25 ◦C). For this pur-
pose, a 1 × 10−5 M solution of these complexes was treated with
1 × 10−3 M (100 equiv.) of 3,5-DTBC and the course of the reaction
was followed by recording the UV–vis spectra of the mixture at
an interval of 5 min. After addition of substrate 3,5-DTBC to the
solutions of the catalysts 1–3, a gradual increase of the band corre-
sponding to 3,5-DTBQ was observed at 400 nm (Fig. 7 for complex
2 and Figs. S1 and S3 in supporting information for complexs 1 and
3, respectively).
hydrogen bonds to phenoxido oxygens O(11) (−x, −y, 2 − z) and
˚
O(31) (−x, −y, 2 − z) with dimensions O· · ·O 2.957(5), 3.040(5) A,
˚
O
H· · ·O 149, 120◦ and H· · ·O 2.22, 2.54 A, respectively. From O(2)
the two hydrogen atoms H(21) and H(22) form hydrogen bonds
to O(41) (1 − x, 1 − y, 1 − z) and O(61) (1 − x, 1 − y,1 − z), respec-
˚
tively with dimensions O· · ·O 2.838(5), 2.929(5) A, O H· · ·O 130,
˚
146, H· · ·O 2.22, 2.17 A, thus forming a 2D supramolecular network.
The structure of 3, a di-phenoxido bridged centrosymmetric
dimer, is shown in Fig. 6. Here the metal atoms are bonded to
the four donor atoms of the ligand in an equatorial plane with
The kinetics of oxidation of 3,5-DTBC were determined by the
method of initial rates and involved monitoring the growth of
Fig. 6. The structure of the centrosymmetric dimer [Mn2L21(NO2)2] in 3 with ellip-
soids at 30% probability. The terminal nitrite O1, N2, O3 is disordered at 50% with a
chloride (not shown).
Fig. 7. Increase of quinone band at 400 nm after addition of 100 fold 3,5- DTBC to
complex 2 (10−5 M) in acetonitrile solvent. The spectra were recorded at an interval
of 5 min.

P. Seth et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 154–161
159
Table 1
Kinetic parameters for oxidation of 3, 5-DTBC catalyzed by various complexes.
Complex
Vmax (M min−1
)
Std. error
KM (M)
Std. error
Kcat (h−1
)
1
2
3
1.606 × 10−4
6.089 × 10−5
2.388 × 10−4
1.374 × 10−5
4.380 × 10−6
9.395 × 10−6
5.501 × 10−4
6.408 × 10−4
4.917 × 10−3
1.506 × 10−6
1.081 × 10−6
3.803 × 10−5
936.64
365.34
1432.74
the quinone band at 400 nm as a function of time. To determine
the dependence of the rates on the substrate concentration and
various kinetic parameters, solutions of 1–3 were treated with
different concentrations of 3,5-DTBC (from 10 to 100 equiv.) under
aerobic conditions. The data were taken at an interval of 30 s
and the conversion was measured up to 10 min in each case.
From these data the rate of the catechol oxidation reaction with
varying substrate concentration was found out. At low concen-
trations of 3,5-DTBC a first-order dependence of the substrate
concentration was observed. At higher concentrations, saturation
kinetics was found for all compounds. The rate constant versus
concentration of substrate data was then analyzed on the basis of
the Michaelis–Menten approach of enzymatic kinetics to obtain
the Lineweaver–Burk (double reciprocal) plot as well as the values
of the various kinetic parameters Vmax, KM, and Kcat. Observed rate
vs. [substrate] plot and the Lineweaver–Burk plot for complex 2
are shown in Fig. 8. Similar plots for 1 and 3 are given in the sup-
porting information (SI Figs. S2 and S4, respectively). The kinetic
parameters for all cases are listed in Table 1. We have also carried
out kinetic experiments by varying the catalyst concentration,
keeping the substrate concentration at 10−3 M, and found a first
order rate dependence for all the three catalysts 1–3 (Fig. 9).
The enzyme catechol oxidase consists of a hydroxobridged
dicopper(II) center and consequently mono or dinuclear copper
complexes are extensively studied as model complexes to exam-
ine catecholase activity. It has been found that structural features
and electrochemical properties are important factors in determin-
ing the catalytic activity of these complexes [34]. The mechanistic
course of catechol oxidation reaction catalyzed by copper(II) com-
plexes has been extensively studied [35,36]. But up to now there has
been only limited study of plausible mechanisms of this reaction
using manganese(II/III) complexes as catalyst [37–39]. However,
there is some ambiguity regarding the participation of metal cen-
tre in the catechol to quinone oxidation process [17]. In order to
understand whether the redox participation of the metal center is
responsible for the catecholase activity established for our Mn(III)
complexes, we have performed an EPR study at 77 K selecting com-
plex 2 as the catalyst. For this, we have prepared a 1:100 mixture
Fig. 9. Plot of rate vs. catalyst concentration for the oxidation reaction of 3,5-DTBC
(10−3 M) catalyzed by the complexes 1–3 in acetonitrile solvent.
of complex 2 and 3,5-DTBC (10−1 M) in acetonitrile solvent and
recorded the EPR spectra within 2 min of mixing. A characteris-
tic six line EPR spectrum (Fig. 10) corroborates the generation of
Mn(II) species from Mn(III) species during the catalytic oxidation
of 3,5-DTBC to 3,5-DTBQ in presence of aerial oxygen. To detect
the percentage of formation of Mn(II) species during this oxidation
reaction, we have utilized the spin-quantitation method [40]. In this
method the double-integrated value of EPR spectra (first-derivative
signal) of the experimental sample is usually compared with a stan-
dardized species whose spin content and concentration is known.
For this purpose we have recorded the EPR spectra of a 1:100 mix-
ture of complex 2 (concentration of Mn(III) is 3.0 × 10−3 M) and
3,5-DTBC and compared the EPR peak intensity with that obtained
from a standard Mn(II) complex of 10−3 M concentration (Fig. S9).
From the double integral values of these two spectra in the range
3000–3600 G (Fig. S10) we have calculated the concentration of
Fig. 10. EPR spectrum of a 1:100 mixture of complex 2 and 3,5-DTBC (10−1 M) in ace-
tonitrile at 77 K. Center field: 3000.000 G; sweep width: 2.5 × 100 mT; microwave
frequency: 9126.324 MHz; mod frequency: 100.00 KHz; mod width: 1.0 × 0.1 mT;
time const.: 0.03.
Fig. 8. Plot of rate vs. substrate concentration for complex 2. Inset shows
Lineweaver–Burk plot.

160
P. Seth et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 154–161
O
2
Electronic supplementary data
O
N
O
3
,
5
Mn(III)
-
D
T
B
O 2
Enzyme kinetics data for 1 and 3, ESI-MS spectra, EPR spec-
tra. Crystal data, bond lengths and bond angles in the metal
coordination spheres of 1–3. CCDC-872032 (for 1), -872033
(for 2), -872034 (for 3) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif.
Q
N
O
-.
O
S
O
O
O
N
O
Mn(III)
Mn(II)
N
N
N
O
O
O
O
O
-.
Mn(III)
Acknowledgments
O
N
N
S
We thank EPSRC and the University of Reading for funds for the
X-Calibur system for data of 1 and 2. We thank DST-FIST, India-
funded Single Crystal Diffractometer Facility at the Department
of Chemistry, University of Calcutta, Kolkata, India for data of 3.
We are also thankful to UGC for Junior Research Fellowship to P.S
(UGC/20/Jr. Fellow (Sc) dated 10.01.2011).
O
O
C
Mn(III)
TB
D
N
N
5-
,
O
OH
3
H2O
S= Solvent
Appendix A. Supplementary data
Scheme 3. Formation of different intermediate species during the course of catechol
oxidation.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2012.08.024
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In this paper we present three Mn(III) complexes of salen-type
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active site of catechol oxidase enzyme. These three complexes
(1–3) exhibit high catalytic activity for the oxidation of 3,5-di-
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